1. Field of Invention
The present invention relates to optical devices. The present invention is more particularly related to optics that increase the efficiency and quality of the optical devices and/or reduce costs in producing the optical devices. The optical devices include, for example, prism assemblies and kernels used in image systems such as LCoS based video projection systems.
2. Discussion of Background
Light Management Systems (LMSs) are utilized in optical devices, particularly projection video devices and generally comprises a light source, condenser, kernel, projection lens, and a display screen, and related electronics. The function of the components of a video projector 100 is explained with reference to FIG. 1. As shown, white light 110 is generated by a light source 105. The light is collected, homogenized and formed into the proper shape by a condenser 115. UV and IR components are eliminated by filters (e.g., hot/cold mirrors 116/117). The white light 110 then enters a prism assembly 150 where it is polarized and broken into red, green and blue polarized light beams (hence, a “three channel” type prism assembly). A set of reflective microdisplays 152A, 152B, and 152C are provided and positioned to correspond to each of the polarized light beams (the prism assembly 150 with the attached microdisplays is called a kernel). The beams then follow different paths within the prism assembly 150 such that each beam is directed to a specific reflective microdisplay. The microdisplay that interacts with (reflects) the green beam displays the green content of a full color video image. The reflected green beam then contains the green content of the full color video image. Similarly for the blue and red microdisplays. On a pixel by pixel basis, the microdisplays modulate and then reflect the colored light beams. The prism assembly 150 then recombines the modulated beams into a modulated white light beam 160 that contains the full color video image. The resultant modulated white light beam 160 then exits the prism assembly 150 and enters a projection lens 165. Finally, the image-containing beam (white light beam 160 has been modulated and now contains the full color image) is projected onto a screen 170.
A number of prism assemblies are commercially available. In most, the configuration of the prism assembly consists of precisely formed optical components that have been bonded together. The specific construction techniques by which this is accomplished provides differing advantages and disadvantages, and the components and arrangements of components within the prism assemblies vary according to their designs.
One challenge in designing a light engine is to produce an image with the blackest possible dark state. One of the means by which this can be accomplished is to insert quarter waveplates between the microdisplays and the faces of the prism. One function of the waveplate is to compensate the residual birefringence that exists in the high voltage (dark) state of the microdisplay.
To obtain the blackest possible dark state, the conventional procedure is as follows:
The retardation values of the quarter waveplates should be matched to the center of the light spectrum of each channel. For example, the retardation of the waveplate in the red channel might be centered at 150 nm, the green at 135 nm and the blue at 110 nm.
The highest available voltage is applied to all three microdisplays (this produces the lowest possible value of residual retardation).
Each waveplate is cut into an “oversized” rectangular piece in which its principle retardation axes are oriented parallel and perpendicular to the edges. The “red” waveplate is placed between the “red” microdisplay and the prism. With the voltage applied to the red microdisplay, the red waveplate is rotated to the angle that produces the blackest dark state in red channel (“tuning” the waveplate). The same procedure is then applied to the green and the blue channels.
Note that the use of “tuned” waveplates is desirable in that they produce the blackest possible dark state. Tuned waveplates are not, however, strictly required. It is possible to use waveplates having arbitrary values in the visible spectra. The axis angle required to obtain the blackest possible dark state with an arbitrary waveplate will, in general, be different from that required for a tuned waveplate. More importantly, although the blackness of the dark state obtained for the arbitrary waveplate can be optimized it will not, in general, be as black as that obtained with a tuned waveplate. However, the blackness difference is likely to be small.
Thus, in the conventional procedure to obtain the blackest possible dark state, the compensating retardation of the waveplate is varied to match the fixed residual retardation of the microdisplay. In theory, this procedure works quite well. In reality, when evaluated for use in a high volume manufacturing environment, the process is found to be difficult, time consuming and expensive to implement.
In addition, in an actual high-volume manufacturing environment, it is found that there are major difficulties in accomplishing proper insertion of the waveplate. These difficulties relate to the physical properties of the quarter waveplate material.
Some waveplates are not flat. This is often the case when the waveplate material is “thick”. If the material is not flat, distortion can be introduced into the focus of the image. In addition, proper lamination of the waveplate (as is required in some configurations) is difficult to accomplish.
Some waveplates are too flexible. This is the case when the waveplate material is too thin, almost like cellophane. Such a material cannot be easily manipulated during the assembly process.
In both the thick and thin varieties of waveplate, it is difficult to obtain materials that are defect free. This is particularly important in that the waveplate is close to the focal plane of the microdisplay. Any defects in the waveplate will almost certainly be in focus and visible in the projected image.
An additional difficulty is that the best commercially available waveplate materials are prohibitively expensive (expensive being defined in the context of a kernel application).
Some light engines are designed to produce polarized light and utilize a kernel that requires the input of polarized light. Other light engines are designed to produce unpolarized light and utilize a kernel that requires the input of unpolarized light.
Some desirable properties of a prism assembly are that it produce an image in which the brightness and color are spatially uniform and meet other required optical properties including those for brightness, color gamut and contrast ratio. It is equally important that the prism assembly be manufacturable. That is, that the prism assembly be producible in high volume with good yield and at a high quality level. Finally, and of equal importance, is that the prism assembly meet cost targets. Targets that, for consumer products, are inevitably very challenging.
Stray light is one factor that limits the contrast ratio produced by an image device, and particularly LCoS based video projection systems. Stray light can be generated within a prism assembly/kernel in several ways including:                Diffraction from the microdisplay.        Non-specular reflection from the surface of the microdisplay.        Reflections from interfaces between the components in the optical path.        Scattering from imperfections on the optical surfaces, in the thin film coatings or within the bulk materials.        
Some of the stray light inevitably exits the kernel to be included in the projected image. The stray light adds light intensity but not content to the video image. In current light engine designs and applications, this can be a significant contrast ratio-limiting factor. Techniques used to suppress stray light include:                The use of picture frame baffles.        Removing light rays that impact and scatter/reflect from the walls of the prism assembly. The removal is accomplished by either absorbing or transmitting the stray light rays.        
However, the present inventors have determined that, in actual light engine applications, these techniques are insufficient. And, the present inventors have realized the need for further and better suppression of stray light.
The LCoS based video projection system also utilizes a number of optical components, such as beam splitting elements, waveplates, dichroics, etc. Each of the optical components contribute to one or more of the technical challenges in producing a high quality low cost optical device.